Ordinary double glazed sealed units normally have a thickness of approximately 20 mm. When better thermal protection is required, thick triple glazed sealed units may be used, which feature air spaces of about 12 mm each. These triple glazed units present excessive thickness for use in normal sashes having a 20 mm rabbet.
As used herein, the term "thick multiple glazed units" designates a sealed glazing assembly of at least two glass panes spaced relative to each other and forming an insulating units having a thickness in the 30 to 40 mm range, and adapted to be mounted in a sash. This expression includes triple glazed units comprising two air spaces of 13 mm each. This particular triple glazed unit turns out to be very efficient and it is increasingly popular. It has a thickness of approximately 38 mm.
With the increasing heating costs, thermal insulation of dwellings has become a major preoccupation. Air infiltration through windows must be reduced, and glass units of improved thermal efficiency are required. Double glazed units have become quite popular but triple glazed units are now generating more than a passing interest due to the increase of heating costs. In recent years, the use of triple glazed units has been in substantial demand in areas of harsh climates.
Thick multiple glazed units, including triple glazed units featuring 13 mm air spaces, provide the required degree of thermal insulation but their excess thickness creates a very serious problem due to insufficient space in the sashes and in the window frames of ordinary construction. For instance, thick triple glazed units do not fit in double glazed sashes on account of insufficient rabbet depth, and this is also true for the jamb. This problem becomes particularly acute in the case of casing windows when room is required to accommodate the swinging of the sash around its hinges. Increasing the sash thickness will result in complete inoperativeness of the window since the sash could not pivot in the jamb.
Thus, one may use thin triple glazed units having two air spaces of reduced thickness as this allows them to be mounted in a sash having a normal thickness, in the usual manner. However, from the standpoint of thermal efficiency, such a solution defeats the main purpose of the change because it greatly reduces the insulation expected of triple glazed units. Such "thin" triple glazing is not really very superior to the usual double glazed units.